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L 21(dp)(pe) ((ee)nptel)

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  • 1. Module 3DC to DC Converters Version 2 EE IIT, Kharagpur 1
  • 2. Lesson 21Introduction to Switched- Mode Power Supply (SMPS) Circuits Version 2 EE IIT, Kharagpur 2
  • 3. After completion of this lesson the reader will be able to: (i) Identify the basic elements in a regulated power supply (ii) Explain the basic principle of operation of linear and switched mode power supplies (iii) Compare the merits and demerits of SMPS vis-à-vis linear power supplies (iv) Interpret Power supply specifications21.1 Introduction to regulated dc power suppliesPower supply is a broad term but this lesson is restricted to discussion of circuits that generate afixed or controllable magnitude dc voltage from the available form of input voltage. Integrated-circuit (IC) chips used in the electronic circuits need standard dc voltage of fixed magnitude.Many of these circuits need well-regulated dc supply for their proper operation. In majority ofthe cases the required voltages are of magnitudes varying between -18 to +18 volts. Someequipment may need multiple output power supplies. For example, in a Personal Computer onemay need 3.3 volt, ±5 volt and ±12 volt power supplies. The digital ICs may need 3.3volt supplyand the hard disk driver or the floppy driver may need ±5 and ±12 volts supplies. The individualoutput voltages from the multiple output power supply may have different current ratings anddifferent voltage regulation requirements. Almost invariably these outputs are isolated dcvoltages where the dc output is ohmically isolated from the input supply. In case of multipleoutput supplies ohmic isolation between two or more outputs may be desired. The inputconnection to these power supplies is often taken from the standard utility power plug point (acvoltage of 115V / 60Hz or 230V / 50Hz). It may not be unusual, though, to have a power supplyworking from any other voltage level which could be of either ac or dc type.There are two broad categories of power supplies: Linear regulated power supply andswitched mode power supply (SMPS). In some cases one may use a combination of switchedmode and linear power supplies to gain some desired advantages of both the types.21.2 Linear regulated power supplyFig. 21.1 shows the basic block for a linear power supply operating from an unregulated dcinput. This kind of unregulated dc voltage is most often derived from the utility ac source. Theutility ac voltage is first stepped down using a utility frequency transformer, then it is rectifiedusing diode rectifier and filtered by placing a capacitor across the rectifier output. The voltageacross the capacitor is still fairly unregulated and is load dependent. The ripple in the capacitorvoltage is not only dependent on the capacitance magnitude but also depends on load and supplyvoltage variations. The unregulated capacitor voltage becomes the input to the linear type powersupply circuit. The filter capacitor size is chosen to optimize the overall cost and volume.However, unless the capacitor is sufficiently large the capacitor voltage may have unacceptablylarge ripple. The representative rectifier and capacitor voltage waveforms, where a 100 volts(peak), 50 Hz ac voltage is rectified and filtered using a capacitor of 1000 micro-farad and fed toa load of 100 ohms is shown in Fig.21.2. For proper operation of the voltage regulator, theinstantaneous value of unregulated input voltage must always be few volts more than the desiredregulated voltage at the output. Thus the ripple across the capacitor voltage (difference betweenthe maximum and minimum instantaneous magnitudes) must not be large or else the minimumvoltage level may fall below the required level for output voltage regulation. The magnitude ofvoltage-ripple across the input capacitor increases with increase in load connected at the output. Version 2 EE IIT, Kharagpur 3
  • 4. The step down transformer talked above should be chosen such that the peak value of rectifiedvoltage is always larger than the sum of bare minimum voltage required at the input of theregulator and the worst-case ripple in the capacitor voltage. Thus the transformer turns ratio ischosen on the basis of minimum specified supply voltage magnitude. The end user of the powersupply will like to have a regulated output voltage (with voltage ripple within some specifiedrange) while the load and supply voltage fluctuations remain within the allowable limit. Toachieve this the unregulated dc voltage is fed to a voltage regulator circuit. The circuit inFig.21.1 shows, schematically, a linear regulator circuit where a transistor is placed in betweenthe unregulated dc voltage and the desired regulated dc output. Difference between theinstantaneous input voltage and the regulated output voltage is blocked across the collector -emitter terminals of the transistor. As discussed previously, in such circuits the lowestinstantaneous magnitude of the unregulated dc voltage must be slightly greater than the desiredoutput voltage (to allow some voltage for transistor biasing circuit). The power dissipation in thetransistor and the useful output power will be in the ratio of voltage drops across the transistorand the load (here the control power dissipated in the base drive circuit of the transistor isassumed to be relatively small and is neglected). The worst-case series voltage drop across thetransistor may be quite large if the allowed variation in supply magnitude is large. Worst-casepower dissipation in the transistor will correspond to maximum supply voltage and maximumload condition (load voltage is assumed to be well regulated). Efficiency of linear voltageregulator circuits will be quite low when supply voltage is on the higher side of the nominalvoltage. Series pass elements Unregulated DC voltage Regulated Output Fig.21.1: Schematic linear voltage regulator Version 2 EE IIT, Kharagpur 4
  • 5. Power- electronic switchUnregulated Duty ratioDC source control + Load Output voltage - feedback Fig. 21.3: A schematic switched mode dc to dc chopper circuitProblem 1An 18V (rms), 50 Hz supply is rectified using a full bridge diode rectifier and is followed by acapacitor filter. The load connected across the capacitor is a simple resistor of 30 ohm. Whatshould be the value of filter capacitor to get only 5 volts peak to peak ripple across the loadvoltage? Neglect voltage drop across conducting diode.[Hint: The exact solution will involve use of numerical technique or trial and error method.However with some simplifying assumptions, fairly accurate value of capacitance may be foundout. It may be assumed that in each half cycle the capacitor charges to the peak of supply voltage(= 18*1.414 =25.456 volts). The ripple in the capacitor voltage may be neglected to calculateload current. Thus capacitor may be assumed to discharge under the influence of 25.456/30 amp. 1 ⎧ − ⎛ 25.456 − 5 ⎞ ⎫( I ) for a time duration (Δt) equal to ⎨π − Cos ⎜ ⎟⎬ . Next, use the equality 2π ( freq.) ⎩ ⎝ 25.456 ⎠⎭C ΔV = I Δt and find C.Answer: C = approx. 1350 microfarad.] Version 2 EE IIT, Kharagpur 5
  • 6. Problem 2It is desired to get a regulated 15 volts supply from the utility ac voltage of 50 Hz using a linearregulator circuit. The input ac voltage (rms magnitude) varies from 190 volts to 260volts. Theutility voltage is first stepped down using a transformer. The stepped down voltage is rectifiedusing a diode bridge and filtered by placing a capacitor after the rectified output. Assuming peak-to-peak ripple in the capacitor voltage to be 10% of the capacitor’s crest voltage, find the turnsratio of the step down transformer. For proper operation of the linear regulator circuit the inputvoltage applied to it must always be 2 volts more than the desired output voltage (neglect diodedrops).[Answer: Turns ratio = L.V. turns/ H.V. turns = {1.11 (15+2) } / (190* 1.414) = 1 : 14]21.3 Switched Mode Power Supply (SMPS)Like a linear power supply, the switched mode power supply too converts the availableunregulated ac or dc input voltage to a regulated dc output voltage. However in case of SMPSwith input supply drawn from the ac mains, the input voltage is first rectified and filtered using acapacitor at the rectifier output. The unregulated dc voltage across the capacitor is then fed to ahigh frequency dc-to-dc converter. Most of the dc-to-dc converters used in SMPS circuits havean intermediate high frequency ac conversion stage to facilitate the use of a high frequencytransformer for voltage scaling and isolation. In contrast, in linear power supplies with inputvoltage drawn from ac mains, the mains voltage is first stepped down (and isolated) to thedesired magnitude using a mains frequency transformer, followed by rectification and filtering.The high frequency transformer used in a SMPS circuit is much smaller in size and weightcompared to the low frequency transformer of the linear power supply circuit.The ‘Switched Mode Power Supply’ owes its name to the dc-to-dc switching converter forconversion from unregulated dc input voltage to regulated dc output voltage. The switchemployed is turned ‘ON’ and ‘OFF’ (referred as switching) at a high frequency. During ‘ON’mode the switch is in saturation mode with negligible voltage drop across the collector andemitter terminals of the switch where as in ‘OFF’ mode the switch is in cut-off mode withnegligible current through the collector and emitter terminals. On the contrary the voltage-regulating switch, in a linear regulator circuit, always remains in the active region.Details of some popular SMPS circuits, with provisions for incorporating high frequencytransformer for voltage scaling and isolation, have been discussed in next few lessons. In thislesson a simplified schematic switching arrangement is described that omits the transformeraction. In fact there are several other switched mode dc-to-dc converter circuits that do not use ahigh frequency transformer. In such SMPS circuits the unregulated input dc voltage is fed to ahigh frequency voltage chopping circuit such that when the chopping circuit (often called dc todc chopper) is in ON state, the unregulated voltage is applied to the output circuit that includesthe load and some filtering circuit. When the chopper is in OFF state, zero magnitude of voltageis applied to the output side. The ON and OFF durations are suitably controlled such that theaverage dc voltage applied to the output circuit equals the desired magnitude of output voltage.The ratio of ON time to cycle time (ON + OFF time) is known as duty ratio of the choppercircuit. A high switching frequency (of the order of 100 KHz) and a fast control over the dutyratio results in application of the desired mean voltage along with ripple voltage of a very highfrequency to the output side, consisting of a low pass filter circuit followed by the load. The high Version 2 EE IIT, Kharagpur 6
  • 7. frequency ripple in voltage is effectively filtered using small values of filter capacitors andinductors. A schematic chopper circuit along with the output filter is shown in Fig.21.3. Someother switched mode power supply circuits work in a slightly different manner than the dc-to-dcchopper circuit discussed above. Details of some of these circuits have been discussed infollowing lessons.21.4 SMPS versus linear power supplyAs discussed above, in a linear regulator circuit the excess voltage from the unregulated dc inputsupply drops across a series element (and hence there is power loss in proportion to this voltagedrop) whereas in switched mode circuit the unregulated portion of the voltage is removed bymodulating the switch duty ratio. The switching losses in modern switches (like: MOSFETs) aremuch less compared to the loss in the linear element.In most of the switched mode power supplies it is possible to insert a high frequency transformerto isolate the output and to scale the output voltage magnitude. In linear power supply theisolation and voltage-scaling transformer can be put only across the low frequency utility supply.The low frequency transformer is very heavy and bulky in comparison to the high frequencytransformer of similar VA rating. Similarly the output voltage filtering circuit, in case of lowfrequency ripples is much bulkier than if the ripple is of high frequency. The switched modecircuit produces ripple of high frequency that can be filtered easily using smaller volume offiltering elements.Linear power supply though more bulky and less efficient has some advantages too whencompared with the switched mode power supply. Generally the control of the linear powersupply circuit is much simpler than that of SMPS circuit. Since there is no high frequencyswitching, the switching related electro-magnetic interference (EMI) is practically absent inlinear power supplies but is of some concern in SMPS circuits. Also, as far as output voltageregulation is concerned the linear power supplies are superior to SMPS. One can more easilymeet tighter specifications on output voltage ripples by using linear power supplies.Problem 3Estimate and compare the size (window area X core area) of the following two transformers: (i) a50 VA, 50Hz, 15V low frequency transformer and (ii) a 50 VA, 100 kHz, 15V high frequencytransformer. Assume sinusoidal voltages. Assume the peak flux density in low frequencytransformer to be 1.5 tesla and in high frequency transformer to be 0.3 tesla. Take identicalvalues for window utilization factor and copper current density.[Hint: VA ratng for a single phase transformer = 2.22 f BmaxAC AW δ KW , where f is supplyfrequency, Bmax is the peak flux density, AC : core area, AW : window area, δ : current density incopper and KW is the window utilization factor.]Answer: Volume (size) of Low frequency transformer will be 400 times higher than that of highfrequency transformer. Version 2 EE IIT, Kharagpur 7
  • 8. 21.5 Hybrid (SMPS followed by linear) power supplyA comparison of linear and switched mode power supplies tells about the advantages anddisadvantages of the two. Linear power supply is highly inefficient if it has to work over largevariations in input voltage, is more bulky because of the use of low frequency transformer andfilter elements (inductors and capacitors). On the other hand linear power supplies give betteroutput voltage regulation. It may sometimes be required to have output voltage regulation similarto the one provided by linear supplies and compactness and better efficiency of a switched modesupply. For this, the linear power supply may be put in tandem with a switched mode supply. Letus consider a case where one needs an isolated and well-regulated 5 volts output while inputpower is drawn from utility supply that has large voltage fluctuation. In such a situation one maygenerate an isolated 7.5 volts from an SMPS and follow it by a 5 volts linear power supply set towork with 7.5 volts input. The input to linear power supply must be few volts more than therequired output (for proper biasing of the switches) and hence SMPS tries to maintain around 7.5volts input. It can be seen that the linear power supply now does not have large input voltagevariation in spite of large variations in the utility rms voltage. The SMPS portion of the powersupply efficiently performs the job of voltage isolation and conversion from widely varyingutility voltage to fairly regulated 7.5 volts dc. Under the given condition it may not be difficultto see that the overall efficiency of this hybrid power supply will lie between that of a SMPS anda linear supply. The overall cost may or may not increase even though two supplies in tandemare used. It is to be kept in mind that to achieve the same output voltage specification by anSMPS circuit alone, the control and filtering circuit may become more costly and complex (thanthe one used in the hybrid power supply unit). Similarly if the linear supply has to be designedfor larger fluctuation in input voltage the component ratings, including heat-sink ratings, will behigher and may cost as much as the hybrid unit.21.6 Multiple output SMPSA single power supply unit may need to output several different voltages. The individual outputvoltages may have different ratings in terms of output current, voltage regulation and ripplevoltages. These outputs may need isolation between them. Generally a common high frequencytransformer links the input and output windings and in spite of output voltage feedback all theoutputs can not have same regulation because of different loads connected to different outputsand hence different ohmic (resistive) drops in the output windings (loads are generally variableand user dependent). Also the coupling between the different secondary windings and theprimary winding may not be same causing different voltage drops across the respective leakageinductances. Barring this mismatch in the voltage drops across the resistances and leakageinductances of the secondary windings their output voltages are in proportional to their turnsratios. The turns ratios are properly chosen to give fairly regulated individual output voltages(even if only one output voltage feedback is used for SMPS switch control). The output thatneeds to have tighter voltage regulation may be used for output voltage feedback. In case anotheroutput needs to have similarly tight regulation then that particular output may be passed throughan additional linear regulator circuit as in the case of hybrid power supply circuit discussed in theprevious section (Sec. 21.5). Version 2 EE IIT, Kharagpur 8
  • 9. 21.7 Resonant Mode Power SuppliesResonant mode power supplies are a variation over SMPS circuits where the switching losses aresignificantly reduced by adapting zero-voltage or zero-current switching techniques. In non-resonant mode SMPS circuits the switches are subjected to hard switching (during hard-switching, both the voltage and current in the switch are of considerable magnitude resulting inlarge instantaneous switching power loss). Efficiency of resonant mode power supplies isgenerally higher than non-resonant mode supplies.21.8 Power supply specificationsPower supplies may have several specifications to be met, including their voltage and currentratings. There may be short time ratings of higher magnitudes of current and continuous ratingsof somewhat lower magnitudes. One needs to specify the tolerable limits on the ripple voltages,short-circuit protection level of current (if any) and the nature of output volt-current curve duringover-current or short circuit (the output voltage magnitude should reduce or fold back towardszero, gradually, depending on the severity of over-current). The fuse requirement (if any) on theinput and the output side may need to be specified. One needs to specify the type of input supply(whether ac or dc) or whether the power supply can work both from ac or dc input voltages.Acceptable range of variation in input voltage magnitude, supply frequency (in case of ac input)are also to be specified. Efficiency, weight and volume are some other important specifications.Some applications require the electro-magnetic compatibility standards to be met. Byelectromagnetic compatibility it is meant that the level of EMI generation by power supplyshould be within tolerable limits and at the same time the power supply should have the ability towork satisfactorily in a limited noisy environment. It is quite common to have output voltageisolation and it is specified in terms of isolation breakdown voltage. In case of multiple powersupplies it needs to be specified whether all the outputs need to be isolated or not and whatshould be the acceptable ripple voltage range for each.In majority of the cases the available source of input power is the alternating type utility voltageof 50 or 60 Hz. The voltage levels commonly used are 115V (common in countries like, USA)and 230 volts (common in India and many of the European countries). Most utility (mains)power supplies are expected to have ± 10% voltage regulation but for additional precaution theSMPS circuits must work even if input voltages have ± 20% variation. Now-a-days universalpower supplies that work satisfactorily and efficiently both on 115 V and 230 V input are quitepopular. These power supplies are very convenient for international travelers who can simplyplug-on their equipments, like laptop computer and shaving machine, without having to paymuch attention on the exact voltage and frequency levels of the utility supply. In contrast someof the other power supplies have a selector switch and the user is required to adjust the switchposition to match the utility voltage. In case user forgets to keep the selector switch at correctposition, the equipment attached may get damaged.21.9 Some common types of SMPS circuitsThere are several different topologies for the switched mode power supply circuits. Somepopular ones are: fly-back, forward, push-pull, C’uk, Sepic, half bridge and H-bridge circuits.Some of these configurations will be discussed in the coming lessons. A particular topology maybe more suitable than others on the basis of one or more performance criterions like cost, Version 2 EE IIT, Kharagpur 9
  • 10. efficiency, overall weight and size, output power, output regulation, voltage ripple etc. All thetopologies listed above are capable of providing isolated voltages by incorporating a highfrequency transformer in the circuit. There are many commercially available power supplycontroller ICs that can readily be used to control the duty ratio of the SMPS switches so that thefinal output is well regulated. Most of these ICs are capable of driving MOSFET type ofswitches. They also provide features like under voltage lock-out, output over-current protectionetc.Problem 4Which among the following power supplies will be most energy-efficient if operated under wideinput voltage variation and at full load: (i) Linear power supply (ii) Switched mode power supply (iii) Switched mode followed by linear power supply (iv) Linear followed by switched mode power supply Answer: (ii)More Quiz problems on Chapter-21(1) A regulated power supply should be able to maintain output voltage within specified limits in spite of (a) Unlimited variation in supply voltage (b) Unlimited variation in load at the output (c) Both (a) and (b) (d) Only for load and supply parameter- variations in the specified range(2) Linear isolated power supplies will generally be superior to switched-mode power supplies (outputting isolated voltage) in respect to the following (a) Higher efficiency in the entire range of load and supply variations (b) Better dynamic regulation of output voltage against supply variations (c) Less volume and weight for identical input and output ratings (d) All the above(3) High frequency transformers are used for output voltage isolation and scaling in the following type of power supplies:- (a) Switched mode power supplies (b) Linear power supplies (c) Hybrid power supplies (d) Both (a) and (c)(4) A 60watt, 15V ± 0.2V, power supply specified to deliver regulated output for input supply (50Hz) variation from 180 volts to 270 volts will be different from another power supply with identical output rating but capable of outputting regulated voltage over input range of 90 volts to 270 volts in respect of:- (a) Less cost (b) More bulky and more costly (c) Higher current rating of power-switch (d) None of the aboveAnswers: (1-d), (2-b), (3-d), (4-a) Version 2 EE IIT, Kharagpur 10

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